Production of gasoline from natural gas

ABSTRACT

ISOMERS ORIGINALLY CONTAINED IN THE NATURAL GASOLINE FRACTION ARE THEN BLENDED WITH THE HYDROGENATED POLYOLEFIN PRODUCT.   A PROCESS FOR THE PRODUCTION OF GASOLINE FROM NATURAL GAS IN WHICH A C2 TO C4 NATURAL GAS LIQUIDS FRACTION IS CONVERTED TO POLYOLEFINS BOILING BELOW ABOUT 400*F. AND THE POLYOLEFINS ARE THEN HYDROGENATED TO FORM GASOLINE BLENDING STOCK. A NATURAL GASOLINE FRACTION COMPRISING MOSTLY C5 TO C7 HYDROCARBONS IS TREATED BY ISOMERIZATION TO CONVERT AT LEAST ABOUT 95% OF THE NORMAL PARAFFINS CONTAINED THEREIN TO ISOPARAFFINS. THE ISOMER PRODUCT AND

United States Patent 3,654,136 PRODUCTION OF GASOLINE FROM NATURAL GAS Daniel Milsom, Hightstowh, N.J., assignor to Cities Service Oil Company, Tulsa, Okla. Filed Nov. 26, 1969, Ser. No. 880,220 Int. Cl. C10g 37/08 US. Cl. 208-71 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the production of gasoline from natural gas and more particularly to treatment of natural gas liquids to produce gasoline suitable for use as motor fuel. Traditionally, gasoline has been refined from crude oil while natural gas liquids have been used to satisfy LP gas markets with some natural gasoline components being blended into refinery motor fuel pools. Generally, the low boiling ranges of natural gas liquid streams (including ethane, propane, normal butane, isobutane and natural gasoline) preclude use of these components directly as motor fuel and have required blending with higher boiling materials for use as motor fuel. In addition, the octane quality of the natural gasoline (C and higher boiling hydrocarbons) is such that this material cannot normally be used in premium fuel. It has now been found that by use of the proper combination treating steps, suitable gasoline and gasoline components can be produced using the C to C natural gas liquids and natural gasoline fractions recovered from natural gas.

According to the present invention, production of gasoline from natural gas is improved by converting a C to 0., natural gas liquids fraction to polyolefins boiling below about 400 F. and then hydrogenating the polyolefins to form gasoline blending stock. In accordance with a preferred embodiment of the invention, further advantages are obtained by treating a natural gasoline fraction to isomerize normal paraffins contained therein to isoparatiins. The isomer product, as well as isomers originally contained in the natural gasoline fraction, may then be blended with the hydrogenated polyolefins referred to above to form a suitable gasoline. Butanes may be separately recovered from the natural gas and added to the gasoline as needed to control vapor pressure. Gasoline produced in this manner is entirely suitable for use as motor fuel and with addition of proper amounts of lead has sufficient octane quality to be usable as premium motor fuel. If preferred, such gasoline may of course be blended with other refinery gasoline stocks.

The accompanying drawing is a somewhat schematic representation of process steps suitable for practicing the invention.

Patented Apr. 4, 1972 The C to C natural gas liquids fraction recovered from natural gas normally consists essentially of saturated C to C hydrocarbons, i.e., ethane propane, isobutane and normal butane. Conversion of this material to polyolefins in accordance with the invention is preferably accomplished by first converting the paraffins to olefins and then subjecting the olefins to polymerization to form the desired polyolefins boiling below about 400 F. The pyrolysis reaction used in practicing the present invention is a conventional type of reaction widely used in commercial processes for the production of ethylene and propylene. Such processes generally involve temperatures between about 1400 F. and 1700 F., approximately atmospheric pressures and reaction times between about 0.7 and about 1.3 seconds. Partial pressures of hydrocarbons are preferably maintained about 10 to 20 p.s.i.a. with steam generally used as a diluent to achieve the desired partial press. The effluent from the pyrolysis reactor is usually quenched to stop the reaction and water or aromatic oil is commonly used as quench media. The degree of conversion of paraflins to ole-fins is not critical but in practicing the present invention is preferably maintained between about and about by varying operating conditions in accordance with known techniques of operation.

Products of the pyrolysis reaction include an olefinrich stream, a tail gas comprising hydrogen and methane, a fuel oil fraction boiling above about 400 F. and an aromatic distillate or dripolene fraction. The tail gas and fuel oil fractions are removed from the process for disposal by any suitable means and the olefin-rich fraction is subjected to polymerization as described below. Unconverted parafiins may be recycled from the polymerization step to the pyrolysis reaction. The dripolene fraction is also preferably treated by hydrogenation to produce a substantially completely hydrogenated gasoline fraction suitable for use as part of the total gasoline product produced by the process. This may conveniently be accomplished by the use of a conventional two-stage hydrogenation treatment but is not considered an essential part of the invention.

As with the pyrolysis step, the polymerization of the olefins formed in the pyrolysis step may be carried out using any conventional polymerization process. A particularly suitable process is the so called bulk acid process using liquid phosphoric acid catalyst. Typical operating conditions for such polymerization processes include temperatures between about 300 and about 500 F., pressures between about 200 anud about 1000 p.s.i.g., space velocities between about 10 and about 70 volumes of feed per hour per volume of catalyst and ratios of acid to hydrocarbons in the reactor of between about 2 and about 5 volumes of acid per volume of hydrocarbon. Conversion of olefins to polyolefins may range up to 99 weight percent (wt. percent) but conversion levels of between about 70 and about 90% are more commonly used and are preferred for practicing the present invention. Acid consumption usually ranges between about 1 and about 4 lbs. of acid per 100 gallons of polymer. In practicing the present invention, the polymerization reaction is operated in accordance with conventional techniques to produce polyolefins boiling in the range below about 400 F.

Hydrogenation of polyolefins to form hydrogenated gasoline blending stock in accordance with the invention is carried out in conventional hydrogenation reactions and is preferably carried out under operating conditions suitable for obtaining between about 65 and about 75% saturation of the hydrogenated product. Up to about 70% saturation, octane levels of the hydrogenated product 1ncrease, whereas above about 70% saturation, octane levels decrease from the levels obtainable at approximately the 70% saturation level. Any conventional hydrogenation process may be used in hydrogenating polyolefins in accordance with the invention. Catalytic hydrogenation process using suitable catalysts such as cobalt, molybdenum, nickel, tungsten or oxides or sulfides of such metals, preferably on suitable bases such as alumina or silica are preferred. Preferred operating conditions involve temperatures between about 400 and about 800 F, pressures between about 300 and about 3000 psig and space velocities between about 0.3 and about 5.0 'volumes of feed per hour per volume of catalyst. As mentioned above, operating conditions are selected with reference to the particular feedstock involved so as to hydrogenate the polyolefins treated to a level of saturation between about 65 and about 75%.

Natural gasoline fractions recovered from natural gas typically comprise mostly C to C hydrocarbons and contain substantial quantities of both straight chain and branched chain material. For use as gasoline, it is desirable that as much as possible of this natural gasoline fraction be in the branched chain form. Accordingly, the natural gasoline fraction recovered from natural gas is preferably treated first to separate isomers from straight chain hydrocarbons and then to isomerize the straight chain hydrocarbons. While any suitable techniques may be used for this purpose, it is preferred that the separation of branched chain from straight chain hydrocarbons be accomplished by the use of molecular sieve material with the straight chain material being isomerized and again passed through the sieve to remove any unconverted straight chain material for return to the isomerization unit. In accordance with a preferred embodiment of the invention, at least about 95% of the straight chain material, i.e., normal parafiins, contained in the natural gasoline fraction is converted to isomers. Both the molecular sieve separation processes and the isomerization processes necessary to accomplish this are well known and need not be described in detail.

In blending finished gasoline from the various blending stocks prepared in accordance with the preferred embodiments of the invention described herein, it is frequently necessary to adjust vapor pressure. This is conveniently done by adding butane to the finished product. Such butane may be obtained from outside sources but is preferably obtained as a fraction of the original natural gas feed to the process. Sufiicient butane is preferably added to adjust the Reid vapor pressure of the finished gasoline product to the desired value which will vary according to the intended end uses of the gasoline. Normally, gasoline for motor fuel is blended to obtain Reid vapor pressures between about 8 and about 15.

For a further understanding of the invention, reference should be had to the accompanying drawing.

In the drawing, natural gas containing natural gas liquids and natural gasoline is introduced to the process through line 12 and is fractionated in a fractionation unit 14 with recovery of methane through line 16, a C to C natural gas liquids fraction through line 18, a natural gasoline fraction through line 20 and a butane fraction through line 22. The fractionation step may, of course, make use of any conventional fractionation equipment such as is normally used for separations of this type.

The C to C natural gas liquids fraction recovered through line 18 is passed to a pyrolysis unit 24 in which it is treated for conversion of paraffins to olefins. Products of the pyrolysis reaction include tail gas removed through line 26, fuel oil product removed through line 28 and an olefin-rich stream which is the principal product of the reaction and may be removed from the pyrolysis unit through line 32. Dripolenes recovered through line 30 may be hydrogenated in a hydrogenation unit 34 using hydrogen introduced through line 36. Tail gas may then be recovered from the hydrogenation unit 34 through line 38 and a hydrogenated gasoline product through line 40.

The olefin-rich stream recovered from the pyrolysis unit through line 32 is polymerized in a polymerization unit 42 to form polyolefins recovered through line 44, the operating conditions of the polymerization unit being adjusted so that the polyolefins boil essentialy below 400 F. Any unconverted parafiins in the feed to the polymerization unit may be recycled to the pyrolysis unit for conversion to olefins. Polyolefins recovered from the polymerization unit 42 are then treated in a hydrogenation unit 46 with hydrogen introduced through line 48 to produce hydrogenated gasoline blending stock recovered through line 50.

The natural gasoline fraction recovered from the fractionation unit 14 through line 20 is treated in a separation unit 52 for removal of straight chain material therefrom, the separation unit 52 preferably uses molecular sieve material to separate straight chain from branched chain material in a conventional manner. Normal paraffins separated from the natural gasoline fraction are passed through line 54 to an isomerization unit 56 with the product from the isomerization unit 56 being recycled to the separation unit 52 through line 58. In this way, at least about and preferably substantially all of the straight chain material in the natural gasoline fraction is converted to isomers. The isomer product from the isomerization unit 56 as well as the isomers originally contained in the natural gasoline fraction may then be recovered from the separation unit 52 through line 60 and blended with the hydrogenated polyolefins from line 50 and the hydrogenated dripolene product from line 40 along with butanes from line 22 to form a finished gasoline product recovered through line 62.

EXAMPLE As an example of how usable gasoline may be produced from natural gas in accordance with the invention, a typical natural gas is separated in the fractionation unit 14 to produce methane, C to C natural gas liquids, butane and natural gasoline as described above. The C to- C natural gas liquids fed to the pyrolysis unit 24 total about 25,000 barrels per day and include 36 volume percent ethane, 28 volume percent propane, 12 volume percent isobutane and 24 volume percent n-butane. Natural gasoline recovered from the fractionation unit 14 through line 20 is treated with 5 angstrom unit molecular sieve material for separation of normal paraffins. The normal paraflins are isomerized in the isomerization unit 56 and the total isomerized natural gasoline stream totalling 12,500 barrels per day is recovered through the conduit 60 as described above. Compositions of the natural gasoline fraction before and after treatment in the separation unit 52 and isomerization unit 56 are as indicated below.

Natural gasoline Natural gasoline Component (wt. percent) before treatment after treatment C3 and lighter 0. 01 0. 01 i-C4 0. 01 0. 61 n-Ci 0. 19 0. 50 i-C5 24. 71 54. 52 n-C 29. 68 0. ()4 1-Cs 13. 77 22. 74 n-Cs. 9. 63 0. 01 i-C1 12. 48 15. 15 n-C 2. 83 0.01 i-Cs-w 6. 01 G. 42 nC o e O. 68

In this particular example, the finished product includes 11,330 barrels per day of hydrogenated polymer blending stock, 585 barrels per day of hydrogenated dripolenes, 12,550 barrels per day of treated natural gasoline and 2,565 barrels per day of butanes added to bring the Reid vapor pressure of the resulting gasoline product to the desired level of 10. The hydrogenated polymer blending stocks, hydrogenated dripolenes and treated natural gasoline fractions recovered through lines 50, 40 and 60,

respectively, have the following properties:

Hydro- Hydro- Isomer genated genated lzed nat. drlpolene polymer gasoline Gravity, API 52. 5 60. 85. Distillation, F.:

I.B-P 102 200 90 131 253 100 50% 181 295 112 90% 295 380 170 El 374 420 250 RON clear 92. 5 95.5 87. 3 RON-P3 nil/gal T E L 97.0 104. 0 87. 3 MO clear 91. 0 87.5 87. 3 MON+3 mL/gal T E L 96.0 94. 5 87. 3 Percent aromat 50 0 2 Percent olefins. 0 0 Percent parafiins 98 While the invention has been described above with respect to preferred embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention.

I claim:

1. An improved process for the production of gasoline from natural gas of the type wherein a natural gas stream is subjected to fractionation in order to separate the natural gas stream into a natural gasoline fraction, a methane frac- 0 6 about 400 F. by subjecting the C to C natural gas liquids to a pyrolysis reaction, recovering an olefin rich fraction from the pyrolysis reaction and polymerizing ole'fins contained in such olefin rich fraction to form the polyolefins boiling below about 400 F.;

(c) hydrogenating the polyolefins;

(d) recovering the normal butanes from the natural gas feed and blending them with the finished gasoline product in amounts necessary to control the vapor pressure of the finished gasoline within desired limits; and

(e) blending the hydrogenated product of step (c) with the natural gasoline of step (a) and the treated gasoline product of step (d) to form the improved blended gasoline.

2. The process of claim 1 in which at least about of the parafiins originally present in the C to C natural gas liquids fraction are converted to polyolefins.

3. The process of claim 2 further comprising recovering a fuel oil product fraction and dripolenes product fraction from the pyrolysis reaction.

4. The process of claim 3 in which the dirpolenes fraction is further treated by hydrogenation to produce an aromatic gasoline product which is blended with the natural gasoline fraction and the hydrogenated polyolefin fraction in order to form the improved blended gasoline product.

References Cited UNITED STATES PATENTS 2,332,563 10/1943 Egloif 208-71 2,371,355 3/1945 'Ross et al. 208-71 2,684,325 7/1954 Deansely 208-71 2,859,173 10/ 1958 Hess et al. 208-93 3,016,344 2/1962 Kirsch 208-93 3,305,476 2/ 1967 York et a1. 208-79 3,494,857 2/1970 McIlvried et al. 208-144 HERBERT LEVINE, Primary Examiner US. Cl. X.'R. 

